A typical positive displacement pump includes a housing having an inlet and an outlet, a pumping chamber in the housing and a positive displacement pumping member mounted for movement in the pumping chamber. The housing has an inlet passage leading from the inlet to the pumping chamber and an outlet passage leading from the pumping chamber to the outlet. Inlet and outlet check valves are provided in the inlet and outlet passages, respectively. The positive displacement pumping member which may include, for example, a diaphragm, a piston, an eccentrically driven roller, etc., is rapidly driven through intake and discharge strokes to pump a fluid from the inlet to the outlet of the pump.
One problem with pumps of this kind occurs when the pump is used to pump an incompressible fluid and it is driven by a motor, such as an electromagnetic actuator, at a high rate of speed. For example, the pump may be used to pump an incompressible liquid, such as water, and be driven through 60 cycles, i.e., 60 intake and 60 discharge strokes, per second. On each intake stroke, the incompressible liquid must flow in the line through the inlet and into the pumping chamber and, on each discharge stroke, the flow of water from the inlet to the pumping chamber is terminated. For a pump being driven at 60 cycles per second, this means that the water flow to the pump must be stopped and started 60 times each second. The inertia of the moving column of water is such that the water column cannot be stopped and started at that rate, and this can result in incomplete filling of the pumping chamber on the intake stroke and consequent reduction in pumping capacity.